Add shell command to gnss_time_update_service

Add the shell command infrastructure and an initial shell command
requested by a partner.

The partner would like a way to kick the GnssTimeUpdateService out of
"sleep" so that faked passive GNSS location times will be observed
during tests.

The new command requires SET_TIME permission.

Manually tested / confirmed to work with:

<Build with gnss_time_update_service enabled>
adb shell dumpsys gnss_time_update_service
adb shell cmd gnss_time_update_service start_gnss_listening
adb shell dumpsys gnss_time_update_service
adb shell cmd gnss_time_update_service start_gnss_listening
adb shell dumpsys gnss_time_update_service

Bug: 236611620
Test: Manual: see above
Test: atest services/tests/servicestests/src/com/android/server/timedetector/GnssTimeUpdateServiceTest.java
Change-Id: I1476a9381e797905d9b2456282cf0eb78d51a304